In recent times, port probes, spam mail and also SIP attacks / SIP fraud have increased massively and it mostly comes from China, Korea and Palestine. If you run Linux as perimeter firewall, blocking those 3 (and any other you like) is fairly easy. This script downloads the respective network lists from ipdeny.com and inserts rules at the start of the INPUT and FORWARD chains, so IPs from any of those networks can neither reach your firewall not any system behind it.